1. Field Of The Invention
This invention relates to well test apparatus and methods for testing fluids from oil and gas wells, and more particularly, to a three-phase well test apparatus and method utilizing a two-phase separator, pumping, mixing and back pressure control for increasing the accuracy of flow measurement of the liquid constituent and using flow rate control to improve the accuracy of measuring flow of the gas constituent.
2. Description Of The Prior Art
After an oil or gas well has been drilled, and throughout its production life, it is usually desirable to determine the quantities of the various constituents of the well fluid which are being produced by the well.
In some previous testing, the well fluid stream is flowed into a separator which separates the fluid into its gas, oil and water constituents. Such an operation is referred to as a "three-phase" well test. The separated constituents are flowed out of the separator in individual lines and the flow rate and volumes of each constituent is measured so that the operator knows the relative amounts of each. Other operations are carried out to insure proper calibration of the oil, gas and water meters. After testing, the fluid is typically recombined for transport to a processing facility or gathering system.
In a simpler test apparatus, the well fluid is only separated into gas and liquid components, and this is referred to as a "two-phase separator" well test. The oil and water form the liquid constituent or component, and the relative amounts of each may be measured by various methods, such as use of a water cut monitor and a liquids meter in conjunction with a net oil computer. The gas flow is measured the same way as in the three-phase test.
A problem with this prior "two-phase separator" well test apparatus is that liquid is flowed out the separator in a series of dump cycles. Between the dump cycles, the liquid may separate into oil and water constituents within the separator so that the liquid is mostly water when it is initially discharged during a dump cycle. This can result in inaccurate data. Further, when no liquid is flowing through the water cut monitor, the liquid can stagnate and cause problems with the sensor therein. The present invention solves these problems by providing pumped recirculation which results in the liquid being mixed and kept substantially homogeneous and also provides fluid flow through the water cut monitor at all times so that the liquid cannot stagnate therein.
Another problem with the prior "two-phase separator" well test is that a back pressure must be maintained in the gas section of the separator to move the liquid out of the separator. This results in an increase in the flowing well head pressure from the well which can cause problems. In the present invention, the liquid is pumped from the separator, and it is not necessary to increase the pressure in the separator to flow the liquid.
In either a two-phase separator or three-phase separator well test, a separation step is often supplemented with chemicals that are thoroughly mixed with the produced fluids upstream of the separator to enhance separation. Without such chemicals, the separation process is dependent upon the mechanical design of the separator vessel (retention time and fluid rate) and its internals (turbulence dampeners, coalescer elements, and fluid/interface level control devices). The measurement technique used in a properly designed well test system should dictate the flow control methods used and overall configuration of the well test equipment set up. For example, the measurement may be continuous or "on-line" with fluid discharged from the separator. This requires that fluid leave the separator at a rate and physical state/quality consistent with good measurement accuracy. The present invention provides such a system.
It is desirable to obtain well test accuracy which is both economical and which meets the needs of production engineers and operators. The present invention is designed to improve the accuracy of prior art two-phase separation tests without the necessity of total replacement of existing equipment.